Since this bug is only shown when you use lxde with LibreOffice,
it may be LibreOffice which is bugged instead of lxpanel.
I agree that the problem may just as likely be in LibreOffice as in LXDE, but
if you go to the LO forum, you find this problem is declared 'SOLVED' there,
by replacing lxpanel with something else.
I take this to mean there won't be any further examination of how
LibreOffice itself might be adjusted to get around the problem.
Also, I don't think I see a 'Force Version' alternative for LibreOffice as
I found for lxpanel. That is, a way to roll-back to an earlier version.